1. Field of the Invention
This invention relates to an abnormality diagnostic system for internal combustion engines, and more particularly to an abnormality diagnostic system which is adapted to detect abnormalities in input/output objects such as an air-fuel ratio feedback control system thereof and an exhaust gas recirculation system of the engine when the engine is in predetermined operating conditions.
2. Prior Art
Conventionally, the method of detecting abnormalities in an internal combustion engine for automotive vehicles, which is equipped with electronic control systems, can be classified into three kinds as follows:
(i) A method which comprises always monitoring an output value from a temperature sensor or a pressure sensor such as an engine coolant temperature (TW) sensor, an intake air temperature (TA) sensor, and an atmospheric pressure (PA) sensor, and detecting whether there is an abnormality in the sensor, based upon the output value (e.g. Japanese Patent Publication (Kokoku) No. 63(1988)-37254);
(ii) A method which detects an abnormality in a timing system of the engine and comprises monitoring a timing pulse signal generated at predetermined crank angles in synchronism with the engine rotation, such as an output pulse signal from a TDC sensor, and detects whether or not there is an abnormality in the timing system, based upon whether or not a pulse is omitted from the timing pulses (e.g. Japanese Provisional Patent Publication (Kokai) No. 58(1983)-27297); and
(iii) A method which comprises carrying out, by means of logical comparison, detection of an abnormality in an oxygen concentration (O.sub.2) sensor, an exhaust gas recirculation system or an air-fuel ratio feedback control system, which require limited abnormality-detecting conditions, when the engine is in predetermined operating conditions (e.g. Japanese Provisional Patent Publication (Kokai) No. 60(1985)-173332).
According to these conventional methods, it is determined that there is an abnormality in the engine when an output from a particular component part such as a sensor or a control variable used in a control system such as the air-fuel ratio feedback control system continuously shows a value falling outside a predetermined allowable range over a predetermined time period or a predetermined number of times. When an abnormality is detected, a predetermined fail-safe action is immediately taken, while turning on a warning lamp mounted on a dashboard in the vehicle compartment to warn the occupant(s) of the occurrence of the abnormality.
However, of the above conventional abnormality-detecting methods, the methods (i) and (ii), which detect abnormalities in sensors, etc. in direct manners, are relatively reliable, while according to the method (iii), which detects abnormality based upon results of a logical comparison, accurate abnormality detection is difficult to carry out under all possible environmental conditions. For example, the engine can be determined to be abnormal due to an incidental external factor, even when the engine is normally operating.
More specifically, for example, when the O.sub.2 sensor is wet with rain water entering the engine room in rainy weather or during traveling at the waterside, it can be erroneously determined that the O.sub.2 sensor is faulty, even when it is normally functioning. In the case of the exhaust gas recirculation system, when dust is temporarily caught in an exhaust gas recirculation valve thereof or when the operating pressure for the exhaust gas recirculation valve is too low during traveling at a high altitude, etc., the valve cannot operate as desired so that the exhaust gas recirculation system can temporarily malfunction. Even in such a case, it can be determined that there is an abnormality in the exhaust gas recirculation system. Further, in an internal combustion engine equipped with an evaporative emission control system having a canister which temporarily stores evaporative fuel from a fuel tank and from which the evaporative fuel is purged into the intake system of the engine to prevent emission of evaporative fuel to the outside of the engine, a control variable used in the air-fuel ratio feedback control system is feedback-controlled to a small value when evaporative fuel is purged in large quantities from the canister. On such an occasion, the value of the control variable can exceed a predetermined abnormality-determining value, resulting in the determination that the air-fuel ratio feedback control system is malfunctioning, even when it is normally functioning.
As mentioned above, the conventional abnormality-detecting methods have the drawback that abnormality detection is carried out even when it should not be carried out, thus failing to achieve a desired abnormality diagnosis. Further, even a kind of abnormality which occurs temporarily or incidentally and is different from abnormalities which should be detected, is unnecessarily detected and displayed. As a result, checking at a service station or at a plant cannot find out the location of the abnormality or the cause of the abnormality, and in the worst case component parts which are considered to be faulty are unnecessarily replaced with new ones.